LEDs, and driver circuits to energize them, are well known. The brightness of an LED is directly related to the current applied. The voltage developed across an LED depends primarily upon the semiconductor design and technology used and upon manufacturing tolerances. Where multiple LEDs are used in close proximity, it is often desirable that they operate at fairly matched light output levels. Various circuits and approaches have been previously developed for achieving uniform illumination of LEDs. But these previously developed techniques have suffered from one or more disadvantages. For example, these techniques may have driver circuits operating very inefficiently. Also, the techniques may require LEDs manufactured to a close tolerance or with matched parameters established by testing. This drives up cost.
Energy efficiency is particularly important in portable devices energized by primary cells, such as the familiar alkaline “battery”.
For convenience in description, identical components have been given the same reference numbers in the various drawings.